Catalytic reforming, using Pt based reforming catalyst, is one of the most important refinery processes in the world. Most refineries have a catalytic reformer, which converts naphtha fractions into high octane reformate.
Reformers come in many types and sizes--from 2000 BPD fixed bed units to moving or swing bed units processing more than 50,000 BPD. Reformers are available with fixed bed reactors, swing bed reactors, or moving bed reactors. Many new units are moving bed reactors, available from UOP, Inc, Des Plaines, Ill.
Reformers generally use mono-metallic catalysts (Pt on a support such as alumina) or bi-metallic catalyst (Pt-Re on a support). Other combinations of Pt and other metals are known.
All reforming catalysts are believed to contain halogen, almost invariably chlorine. Chlorine is now ubiquitous in catalytic reforming. Chloroplatinic acid may be used in the impregnation solution forming the catalyst. Some refiners may add chlorine coumpounds during normal operation.
One major oil company developed a Pt reforming catalyst regeneration or "rejuvenation" procedure which conducted at least some portions of the regeneration in the presence of one or more chlorine compounds. The procedure was believed originally developed for swing reactor systems which were regenerated every day or so, but this regeneration method, or some variant of it, was eventually used in semi-regenerative reformers and in moving bed reformers.
All of this chlorine can, and does, find its way into gas and liquid products from the reformer. Based on a review of several decades of The Oil and Gas Journal, the key to successful catalytic reforming is lots of chloride. For decades refiners have talked about the problems of getting enough chlorides into the system, and dealing with the chlorides in the gas and liquid products from the reformer.
In 1977 there was talk of the need for heat, chloride and moisture to redistribute platinum.
In 1980 there was a discussion of deposits of ammonium chloride in catalytic reforming compressor internals.
In 1985 there was discussion of the need for, and difficulty of maintaining, 1.0 wt % chloride on bimetallic catalyst between regenerations. It was suggested to "come out on the high side on chloride. "
In Alumina adsorbents effectively remove HCl from reformer H.sub.1 gas stream, Janke et al, Oil and Gas Journal, May 12, 1986, page 64, talked about controlled injection of organic chloride at the reformer reactor inlet, and the mischief caused by all this chloride. The problem was worse with continuous catalytic reforming processes, which were reported "to require higher levels of chloride addition for regeneration . . . " The solution proposed by the authors was use of alumina adsorbents to remove the HCl from the net off gas. This article is incorporated by reference.
In Apr. 1, 1994 there was a discussion of the problem of corrosion in fired heaters due to chloride in the hydrogen from the reforming unit. The proposed solution was to install alumina treaters.
The problem is not limited to reformers. Similar problems occur in some isomerization units, and may occur in other units which are relatively dry and use a chloride containing catalyst.
The conditions which lead to chloride problems are catalysts which contain, or reaction conditions which require, chlorine compounds, and reactants which are dry enough that no separate aqueous phase forms in the vapor/liquid separator downstream of the reactor. Essentially all Pt reformers meet these conditions, and many isomerization and other processing units meet these conditions.
The situation could be summarized as follows for Pt reformers. Although refiners may use different reforming catalysts, all the catalysts seem to contain chlorine. There is enough chlorine either present in the virgin catalyst, or from chlorine addition during reformer operation, or from chlorine added during the catalyst regeneration, so that chlorine compounds appear in all the product streams coming from the reformer. Both gas and liquid products have chlorine compounds.
The raw liquid reformate has chlorides. The net hydrogen gas make has chlorine compounds. When the raw reformate is fractionated, usually in a debutanizer, the overhead vapor fraction contains chlorine compounds.
While chlorides in liquid reformate are a serious problem, the present invention is not directed to solving that problem. Instead, the present invention focusses on removal of chlorides or other acidic halogens present in dry gas streams such as gas streams from a reformer. Of primary concern is removal of chlorides from the net gas make from the reformer vapor liquid separator, the hydrogen rich gas removed from the reformer for use in other refinery processes.
In reforming units with recontacting drums for recycle gas, it would help if some means were available to remove chlorides from recycle gas intermediate the first vapor/liquid separator and the downstream recontacting drum. There is an equilibrium between chlorides in reformate and chlorides in the gas phase, and removing chlorides from recycle gas upstream of the recontacting drum would reduce the amount of chlorides in the liquid reformate removed from the recontacting drum, as well as reducing the amount of chlorides in the recycle gas.
Another concern is removal of chlorides from gas streams generated by downstream processing of raw reformate, e.g., removing chlorides from overhead separator vapor associated with reformate fractionators.
Thus the process of the present invention focusses on removal of chlorides from gas streams, rather than from liquid streams.
It should also be clarified that while most reformers use chlorines as a catalyst component, some may use other halogens, such as F or I, but Cl is the halogen of choice, so hereafter chlorine and its reaction or degradation products will be referred to rather than halogens in general.
To solve the problem of removing chlorides from gas streams, refiners have generally used beds of solid adsorbents, such as alumina impregnated with an alkaline material such as NaOH. Such approaches are discussed in the 1994 and 1986 OGJ articles discussed above. While these approaches work, there are problems associated such alumina beds. The problems can include one or more of: cost, catalytic activity, regeneration and disposal.
Alumina beds are relatively costly, in terms of the amount of active ingredient present. The alumina material typically contains 5 to 10 wt % caustic. Alumina costs much more than caustic, and the alumina primarily serves as a support, but one which unfortunately is not always inert.
Alumina beds can exhibit catalytic activity. When alumina beds are used to remove chlorides from flowing gas streams, aluminum chloride can form, and cause catalytic reactions which convert or polymerize some of the hydrocarbon gas species into a much higher molecular weight material. In some units, the gas is turned to go, at least enough is formed that the effectiveness of the alumina bed is much impaired. This heavy viscous material must be removed to "strip" the alumina bed, so that it may be used to absorb additional amounts of chlorides or other acidic components from the flowing gas stream.
Steam stripping will "strip" such a bed, but tends to form wet HCl, which can and has attacked the metal. The HCl formed by steam stripping creates another possible emission problem. Additionally, the water produced when the steam condenses contains benzene, so the water has to be treated for benzene removal before the water can be discarded.
Finally, disposal of solid adsorbents after they are exhausted in use can be a serious waste management problem. Solid bed adsorbents must eventually be retired and the bed frequently contains too much hydrocarbon, and frequently contains too much benzene, to permit the material to be dumped into a landfill. The adsorbent bed may be steam stripped as a prelude to disposal. The resulting water/hydrocarbon product must be stripped to remove benzene from the waste water. The benzene and lighter hydrocarbons removed from the waste water are usually incinerated, and some chlorides may be present in these streams and cause problems during during incineration.
The problems of stripping alumina treaters, in reformers at least, are so severe that some refiners have resorted to expensive treatments with hot inert gas to remove liquids.
I studied the problem of chloride removal from dry gas streams and realized that much of the problem could be overcome by a different approach, which ignored much of the conventional wisdom in gas treating.
While I still made use of a simple acid/base neutralization reaction, my approach used concentrated solid caustic, rather than caustic on some form of support. Rather than use finely divided solid caustic--which one would intuitively think would be better for gas/solid contact--I used low surface area pellets, of large and uniform size and shape and having relatively little porosity inside the pellets.
I developed a way to treat hot or cool bone dry gases in a totally dry process. This aspect of my process could be used to produce salt as essentially the only product of the neutralization reaction.
This new approach to gas treating allowed significant modifications to some refinery processes. In treating reformer recycle gas I was able to remove a significant amount of the chloride present in the recycle gas. Much of the chloride that the reformer feed, or the reformate "sees" is the chloride present in the recycle gas. Because of the way many modern reformers run, with an initial vapor/liquid separator followed by a recontacting drum operating at higher pressure, the gas phase in contact with reformate in the recontacting drum (the V/L separator downstream of the recycle gas compressor) might be called the recontacting gas.
This recontacting gas, or recycle gas, is almost invariably present in large molar excess, relative to feed. Most reformers operate with 2:1 to 5:1 or even higher molar ratios of hydrogen to hydrocarbon. The recycle gas outnumbers the feed, on a molar basis. Removing much of the fugitive chloride from the recycle gas could reduce the chloride loading of the reformate. There could still be some chloride in reformate, due to extraction of chlorides from the reforming catalyst, but the problem would be reduced. This could reduce the amount of chloride in the liquid reformate stream, as well as reduce chlorides in the net gas make of the reformer.
The process could also be used to treat only the net gas make, or excess recycle gas make which is removed as one of the gas phase products of the platinum reformer. While this stream usually is not considered corrosive (it typically has less than 10 ppm water and only a few ppm chlorides) the catalytic uses to which this hydrogen rich stream is sometimes put can make the chloride content a significant problem.
The debutanizer overhead gas make from a reformer may also be treated. Although a relatively small gas stream, it typically has a higher chloride content than any other gas stream associated with the reformer, and usually must be treated for chloride removal before use as fuel or in other refinery processes.
The new approach to gas treating required a new use of an old material, and ignoring much of the art of gas treating.
Conventional wisdom said that very high surface areas were needed. The obvious approach was to disperse the caustic on a porous support, such as alumina. This better dispersed the caustic, and provided a solid support that would not collapse as the caustic was consumed in the neutralization reaction. Although many refiners use this approach, it is costly and can create problems (polymerization reactions) and disposal of spent absorbent.
I tried achieving high surface area mechanically, by simply grinding up large beads of caustic. This produced a bed of crushed caustic which efficiently removed chlorides for a short time while the bed rapidly plugged. It plugged quickly with upflow through the bed, and plugged just as quickly with downflow.
The conventional wisdom on acid base reactions is that they proceed rapidly when aqueous solutions are involved but slowly, if at all, when dry solid:gas interactions are involved. A review of Perry's Chemical Engineer's Handbook, Sixth Edition, shows many neutralization reactions involving aqueous phases, but little on solid:gas chemical reactions. There is a section, 20-72 through 20-74 which is incorporated by reference, on circulating fluid bed combustors (called CFBC units by many workers in this area) which burn relatively small amounts of coal in the presence of larger amounts of limestone or dolomite. At the temperatures involved, reported to be 800.degree. to 900.degree. C., sulfur compounds in the coal form SO.sub.2 which reacts with limestone or dolomite to form CaSO.sub.4.
There is also a discussion of using limestone for disposal of toxic or hazardous wastes by chemical capture or complete destruction. Contact times of 5 to 10 seconds at 900.degree. to 1000.degree. C. were reported to completely destroy most compounds. Limestone reacted with halides, sulfides, metals, etc. to form stable compounds which could be landfilled.
Thus it was known from the CFBC work that large amounts of dolomite or lime could remove small amounts of SOx and other acidic components in a circulating fluidized bed at 800.degree. to 1000.degree. C. This was not helpful for refinery and petrochemical plants which do not have such a high gas temperature, and or can not afford the large capital expense needed to install a circulating fluidized bed of limestone and dispose of the large quantity of spent adsorbent generated.
Many refinery streams are bone dry. Many operate at a temperature above that which permits aqueous scrubbing at low pressure, but at temperatures below those found in CFBC units or the 900.degree. to 1000.degree. C. used in disposal of toxic wastes. Even when gas stream temperatures are low enough to permit existence of an aqueous phase refiners may not want to have an aqueous phase around, as this greatly complicates the design of the gas treating reactor. Wet gas streams also frequently have to be dried before recycling or reuse in other hydroprocessing units.
I discovered that solid caustic could be used to recover acidic species such as halides from dry gas. The key to the process was to avoid most of the things engineers typically believe are required for effective contact and to ignore some of the conventional wisdom regards acid base reactions.
Rather than use a bed of finely divided material, with a large volume of internal pore structure, I did the opposite. I used relatively large particles of solid caustic, which were essentially nonporous. Use of relatively large beads of solid caustic provided a solid bed of mechanically strong, low surface area caustic. Use of uniform sized particles, of regular shape, gave a large void volume which would generally be thought to reduce the effectiveness of the bed at contacting a gas stream.
Closely related to this apparent step backward in bed design (low surface area) I had to ignore the conventional belief that dry gas streams at low temperature would not react at any significant rate with halides in gas streams. I found that refinery streams which were bone dry, and at ambient temperature, could be made to react with solid caustic. Some of my co-workers laughed at me when they first heard of my approach to gas treating, involving low temperatures and bone-dry gas streams.
I discovered that chlorides could effectively be removed from such gas streams by contact with large particles of non-porous solid caustic. Provided I kept the gas dry enough, the chlorides formed salt deposits on the surface of the solid caustic, and the bed did not plug. Despite my co-workers misgivings, my data showed that the process could form soft, high surface area salt deposits on large beads of low surface area solid caustic. Close inspection of the morphology indicated that the growth of fine salt deposits on the caustic is similar to iron rust on steel balls.
Fortuitously, I also developed ways to remove these salt deposits, either mechanically, or by a new, liquid hydrocarbon phase regeneration technique which could restore the chloride removal efficiency of the caustic bed. These regeneration techniques are not necessary for the practice of the present invention, although they may be used very effectively with the process of the present invention to prolong the useful life of the solid caustics.